dmabufs can have a format modifier (DRM_FORMAT_MOD_*) which is used for
tiled layouts for example.  Add a field to QemuDmaBuf so we can carry
around that information.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190529072144.26737-2-kraxel@redhat.com